You can try the scalefnt package and scale it to what you need.

\documentclass[11pt]{article} % use larger type; default would be 10pt
\usepackage{amsmath,scalefnt}

\begin{document}
\scalefont{0.9}{
\begin{gather*}
    A = \left[\,\begin{matrix} a & b & c \\ d & e & f \\ g & h & i \end{matrix}\,\right]
     \        A = \left[\,\begin{matrix} a & b & c \\ d & e & f \\ g & h & i \end{matrix}\,\right]
\end{gather*}}

\end{document}

Edit:

You can adjust the spacing between the matrix elements by using the math kern \mkern, as per the following example:

\[
     A = \scalefont{0.75}{\left[\,\begin{matrix} a&\mkern-15mub&\mkern-15muc\\d&\mkern-15mue&\mkern-15muf\\\end{matrix}\,\right]}
\]

You can try the scalefnt package and scale it to what you need.

\documentclass[11pt]{article} % use larger type; default would be 10pt
\usepackage{amsmath,scalefnt}

\begin{document}
\scalefont{0.9}{
\begin{gather*}
    A = \left[\,\begin{matrix} a & b & c \\ d & e & f \\ g & h & i \end{matrix}\,\right]
     \\
    A = \left[\,\begin{matrix} a & b & c \\ d & e & f \\ g & h & i \end{matrix}\,\right]
\end{gather*}}

\end{document}

Edit:

You can adjust the spacing between the matrix elements by using the math kern \mkern, as per the following example:

\[
     A = \scalefont{0.75}{\left[\,\begin{matrix} a&\mkern-15mub&\mkern-15muc\\d&\mkern-15mue&\mkern-15muf\\\end{matrix}\,\right]}
\]

You can try the scalefnt package and scale it to what you need.

\documentclass[11pt]{article} % use larger type; default would be 10pt
\usepackage{amsmath,scalefnt}

\begin{document}
\scalefont{0.9}{
\begin{gather*}
    A = \left[\,\begin{matrix} a & b & c \\ d & e & f \\ g & h & i \end{matrix}\,\right]
     \        A = \left[\,\begin{matrix} a & b & c \\ d & e & f \\ g & h & i \end{matrix}\,\right]
\end{gather*}}

\end{document}

You can try the scalefnt package and scale it to what you need.

\documentclass[11pt]{article} % use larger type; default would be 10pt
\usepackage{amsmath,scalefnt}

\begin{document}
\scalefont{0.9}{
\begin{gather*}
    A = \left[\,\begin{matrix} a & b & c \\ d & e & f \\ g & h & i \end{matrix}\,\right]
     \        A = \left[\,\begin{matrix} a & b & c \\ d & e & f \\ g & h & i \end{matrix}\,\right]
\end{gather*}}

\end{document}

Edit:

You can adjust the spacing between the matrix elements by using the math kern \mkern, as per the following example:

\[
     A = \scalefont{0.75}{\left[\,\begin{matrix} a&\mkern-15mub&\mkern-15muc\\d&\mkern-15mue&\mkern-15muf\\\end{matrix}\,\right]}
\]